Improved method for manufacturing a structure component for a motor vehicle body

ABSTRACT

A method for manufacturing a rolled product for automobile bodywork or body structure with an alloy containing Si: 0.75-1.10, Fe: max 0.4, Cu: 0.5-0.8, Mn: 0.1-0.4, Mg: 0.75-1, Ti: max 0.15, Cr: max 0.1 and V: max 0.1 is disclosed with several process steps from casting the ingot to forming and painting a car body part. The various possibilities of pre aging of the sheet as well as of the heat treatment of the part offer advantageous material properties in forming, material strength and low sensitivity to the bake hardening process which can vary depending in the part location in the car body.

FIELD OF THE INVENTION

The invention relates to the field of motor vehicle structure parts orcomponents, also referred to as “body in white”, manufactured inparticular by stamping aluminium alloy sheets, more particularly alloysin the AA6xxx series in accordance with the designation of the AluminiumAssociation, intended to absorb energy irreversibly at the time of animpact, and having excellent compromise between high mechanical strengthand good behaviour in a crash, such as in particular impact absorbers or“crashboxes”, reinforcement parts, linings, or other bodywork structureparts.

More precisely, the invention relates to the manufacture of suchcomponents by stamping in a solution-hardened, quenched and naturallyaged temper state followed by hardening by on-part ageing and atreatment of baking the paint or “bake hardening”.

PRIOR ART

Aluminium alloys are increasingly used in automobile construction inorder to reduce the weight of the vehicles and thus reduce fuelconsumption and discharges of greenhouse gases.

Aluminium alloy sheets are used in particular for manufacturing manyparts of the “body in white”, among which there are bodywork skin parts(or external bodywork panels) such as the front wings, roofs, bonnet,boot or door skins, and the lining parts or bodywork structurecomponents such as for example door, bonnet, tailgate or roof linings orreinforcements, or spars, bulkheads, load-bearing floors, tunnels andfront, middle and rear pillars, and finally the impact absorbers or“crashboxes”.

If numerous skin parts are already produced from aluminium alloy sheets,the transposition of steel to aluminium of lining or structure partshaving complex geometries proves to be trickier. Firstly, because of theless good formability of aluminium alloys compared with steels andsecondly because of the mechanical properties that are in generalinferior to those of steels used for this type of part.

This is because this type of application requires a set of properties,sometimes conflicting, such as:

-   -   high formability in the delivery temper, temper T4, in        particular for stamping operations,    -   a controlled tensile yield strength at the delivery condition of        the sheet in order to master the spring back when shaping,    -   good behaviour in the various assembly methods used in        automobile bodywork such as spot welding, laser welding,        adhesive bonding, clinching or riveting,    -   high mechanical strength after cataphoresis and baking of the        paint in order to obtain good mechanical strength in service        while minimising the weight of the part,    -   good energy absorption capacity in the event of impact for        application to bodywork structure parts,    -   good resistance to corrosion, in particular intergranular        corrosion, stress corrosion and filiform corrosion of the        finished part,    -   compatibility with the requirements for recycling of        manufacturing waste or recycled vehicles,    -   acceptable cost of mass production.

There do however now exist mass-produced motor vehicles having a body inwhite consisting mainly of aluminium alloys. For example, the Ford F-150model 2014 version consists of AA6111 structure alloy. This alloy wasdeveloped by the Alcan group in the years 1980-1990. Two referencesdescribe this development work:

-   -   P. E. Fortin et al, “An optimized Al alloy for auto body sheet        applications”, SAE technical conference, March 1984, describes        the following composition: Si: 0.85; Fe: 0.20; Cu: 0.75; Mn:        0.20 and Mg: 0.72.    -   M. J. Bull et al, “Al sheet alloys for structural and skin        applications”, 25th ISATA symposium, Paper 920669, June 1992.

The main property remains high mechanical strength, even if it isinitially designed to withstand indentation for applications of the skintype: “A yield-strength of 280 MPa is achieved after 2% pre-strain and30 min at 177° C.”.

Moreover, other alloys in the AA6xxx family with high mechanicalcharacteristics have been developed for aeronautical or automobileapplications. Thus the alloy of the type AA6056, the development ofwhich dates from the 1980s at Pechiney, has been the subject of manyworks and numerous publications, either to optimise the mechanicalproperties or to improve the resistance to intergranular corrosion. Thiswas the subject of a patent application (WO 2004/113579 A1).

Alloys of the AA6013 type have also been the subject of numerous works.For example, at Alcoa, in the application US 2002/039664 published in2002, an alloy comprising 0.6-1.15% Si; 0.6-1% Cu; 0.8-1.2% Mg;0.55-0.86% Zn; less than 0.1% Mn; 0.2-0.3% Cr and approximately 0.2% Fe,used in the T6 temper, combines good resistance to intergranularcorrosion and an Rp_(0.2) of 380 MPa.

At Aleris, an application published in 2003, WO 03006697, relates to analloy in the AA6xxx series with 0.2% to 0.45% Cu. The object of theinvention is to propose an alloy of the AA6013 type with a reduced Culevel, targeting 355 MPa of Rm in the T6 temper and good resistance tointergranular corrosion. The composition claimed is as follows: 0.8-1.3%Si, 0.2-0.45% Cu; 0.5-1.1% Mn; 0.45-0.1% Mg.

Structural parts for an automobile application made from a 7xxx alloy asdescribed for example in the application EP 2 581 218 are also known.

Furthermore, for producing parts with a complex geometry from aluminiumalloy, such as for example a door lining, which cannot be achieved byconventional stamping with the aforementioned alloys, various solutionshave been envisaged and/or implemented in the past:

-   -   Getting round the difficulty relating to stamping by producing        this type of part by moulding and in particular of the        “under-pressure” type. The patent EP 1 305 179 B1 of Nothelfer        GmbH under priority of 2000 testifies to this.    -   Carrying out a so-called “warm” stamping to benefit from better        suitability for forming. This consists of heating the aluminium        alloy blank, totally or locally, to a so-called intermediate        temperature, that is to say 150° to 350° C., in order to improve        its behaviour under the press, the tool of which may also be        preheated. The patent EP 1 601 478 B1 of the applicant, under        priority of 2003, is based on this solution.    -   Modifying, via its composition, the suitability for stamping of        the alloy in the AA5xxx series itself; it has in particular been        proposed to increase the magnesium content beyond 5%. But this        is not neutral in terms of corrosion resistance.    -   Using composite sheets consisting of an alloy core in the AA5xxx        series, with an Mg content beyond 5% for better formability, and        a clad sheet made from an alloy better resisting corrosion.        However, the corrosion resistance at the edges of the sheet, in        punched zones or more generally where the core is exposed, and        particularly in assemblies, may then prove to be insufficient.    -   Moreover, the document EP 1702995 A1 describes a method for        producing a sheet of aluminium alloy, which comprises the supply        of a molten aluminium alloy having a chemical composition, as a        percentage by weight, Mg: 0.30 to 1.00%, Si: 0.30 to 1.20%, Fe:        0.05 to 0.50%, Mn: 0.05 to 0.50%, Ti: 0.005 to 0.10%, optionally        one or more from among Cu: 0.05 to 0.70% and Zr: 0.05 to 0.40%,        and the remainder: Al and unavoidable impurities: the casting of        the molten alloy in a plate having a thickness of 5 to 15 mm by        the double-strip casting method with a cooling rate at ¼ of the        thickness of the plate of 40° to 150° C./s, coiling in the form        of a reel, homogenisation treatment, cooling of the resulting        reel to a temperature of 250° C. at least at a cooling rate of        500° C./h or more, followed by cold rolling, and then solution        heat treatment. This document does not mention on-part ageing        after forming.    -   WO2018/185425 invention relates to a method for producing a        stamped component of motor vehicle bodywork or body structure        from aluminium alloy comprising the steps of producing a metal        sheet or strip of thickness between 1.0 and 3.5 mm in an alloy        of composition (% by weight): Si: 0.60-0.85; Fe: 0.05-0.25; Cu:        0.05-0.30; Mn: 0.05-0.30; Mg: 0.50-1.00; Ti: 0.02-0.10; V:        0.00-0.10 with Ti+V≤0.10. other elements each <0.05, and <0.15        in total, with the remainder aluminium, with Mg<−2.67×Si+2.87,        dissolving and steeping, pre-tempering, maturation for between        72 hours and 6 months, stamping, tempering at a temperature of        around 205° C. with a hold time between 30 and 170 minutes or        tempering at a time-temperature equivalent, painting and “bake        hardening” of the paints at a temperature of 150 to 190° C. for        15 to 30 minutes. The invention also relates to a stamped        component of motor vehicle bodywork or body structure, also        called a “body in white” produced by such a method.

US20180119261 described 6xxx series aluminum alloys with unexpectedproperties and novel methods of producing such aluminum alloys. Thealuminum alloys are highly formable and exhibit high strength. Thealloys are produced by continuous casting and can be hot rolled to afinal gauge and/or a final temper. The alloys can be used in automotive,transportation, industrial, and electronics applications, just to name afew.

US20180171452 disclosed high-strength, highly deformable aluminum alloysand methods of making and processing such alloys. More particularly,disclosed is a heat treatable aluminum alloy exhibiting improvedmechanical strength and formability. The processing method includescasting, homogenizing, hot rolling, solutionizing, pre-ageing and insome cases pre-straining. In some cases, the processing steps canfurther include cold rolling and/or heat treating.

Having regard to the increasing development of the use of aluminiumsheets for automobile bodywork components and mass production, therestill exists a demand for further improved grades making it possible toreduce thicknesses without impairing the other properties so as alwaysto increase lightening.

Problem Posed

The invention aims to obtain an excellent compromise between formabilityin T4 temper and high mechanical strength as well as good behaviour ofthe finished component under riveting and in a crash, by proposing amethod for manufacturing such components including forming in T4 temperafter natural ageing at ambient temperature, followed optionally by agehardening on the formed part and baking of the paints or bake hardening.One problem is also to achieve a short and economically advantageousmethod and to improve compared to a product made of alloy AA 6111.

These components must also have very good corrosion resistance and goodbehaviour in the various assembly processes such as spot welding, laserwelding, adhesive bonding, clinching or riveting.

OBJECT OF THE INVENTION

-   -   An object of the invention is a method for manufacturing a        rolled product for automobile bodywork or body structure, also        referred to as “body in white”, from an aluminium alloy,        comprising the following successive steps:        -   a. casting of an ingot with the following composition (% by            weight):            -   Si: 0.75-1.10;            -   Fe: max 0.4;            -   Cu: 0.5-0.8;            -   Mn: 0.1-0.4;            -   Mg: 0.75-1;            -   Ti: max 0.15;            -   Cr: max 0.1;            -   V: max 0.1;                inevitable elements and impurities at maximum 0.05%                each, and total 0.15% maximum; remainder aluminium,    -   b. homogenization of the ingot,    -   c. hot rolling of the ingot,    -   d. cold rolling into a sheet,    -   e. solution heat treatment, quenching of the sheet,    -   f. pre ageing of the sheet,    -   g. natural ageing of the sheet.

Another object of the invention is a rolled product obtainable by themethod of the invention.

Another object of the invention is a part obtainable by the method ofthe invention.

Another object of the invention is the use of the part in in a car asbodywork skin parts (or external bodywork panels) such as the frontwings, roofs, bonnet, boot or door skins, and the lining parts orbodywork structure components such as for example door, bonnet, tailgateor roof linings or reinforcements, or spars, bulkheads, load-bearingfloors, tunnels and front, middle and rear pillars, and finally theimpact absorbers or “crashboxes”.

DESCRIPTION OF THE FIGS

FIG. 1 depicts the device for “three-point bending test” consisting oftwo rollers R, and a punch B of radius r, for carrying out the bendingof the rolled product T of thickness t.

FIG. 2 depicts the rolled product T after the “three-point bending” testwith the internal angle β and the external angle, the measured result ofthe test: a is reported in the enclosed result. The maximum strengthduring the test procedure is also reported.

FIG. 3 depicts a specific embodiment for the method:

-   -   1: uncoiler    -   2: coiler    -   3: sheet    -   4: solutionizing furnace    -   5: quenching unit    -   6: surface treatment machine    -   7: pre ageing oven    -   8: stored coil

DESCRIPTION OF THE INVENTION

Unless defined otherwise within this description, the general terms aredefined is the NF EN 12258-1. A sheet is a flat rolled product ofrectangular cross-section with uniform thickness between 0.20 mm and 6mm.

All aluminium alloys in question hereinafter are, unless indicated tothe contrary, designated by the designations defined by the AluminiumAssociation in the Registration Record Series that it publishesregularly.

All the indications relating to the chemical composition of the alloysare expressed as a percentage by weight based on the total weight of thealloy.

The definitions of the metallurgical temper are indicated in theEuropean standard EN 515 unless defined otherwise herein.

The static tensile mechanical characteristics, in other words theultimate tensile strength R_(m), the tensile yield strength at 0.2%elongation Rp_(0.2), and the elongation at break A %, are determined bya tensile test in accordance with NF EN ISO 6892-1.

The bending angles are determined by a three-point bending test inaccordance with NF EN ISO 7438 and the procedures VDA 238-100 and VDA239-200.

The bendability is also measured with the norm ASTM E290-97a.

The inventors selected a set of composition of aluminium alloys inconjunction with suitable methods which offer to car manufacturerinteresting properties to produce parts.

The subject of the invention is a method for manufacturing a rolledproduct for automobile bodywork or body structure, also referred to as“body in white”, from aluminium alloy, comprising the following steps.The casting of an ingot with the following composition: (% by weight):

-   -   Si: 0.75-1.10. preferably the Si content maximum is 1.0% and        more preferably, the maximum Si content is 0.95%.    -   Fe: max 0.4. Preferably the minimum Fe content is 0.15% and/or        the maximum Fe content is 0.30%.    -   Cu: 0.5-0.8. Preferably, the Cu maximum content of the ingot is        0.70% and/or the Cu minimum content is 0.55%. More preferably,        the maximum Cu content is 0.65%. Limiting the Cu to 0.8%, 0.70%        or even 0.65% is interesting for economical reason as Cu is        usually more expensive than aluminium. It is also advantageous        to ease recyclability of the material. It may also improve the        corrosion resistance. In another embodiment however the    -   Cu minimum content is 0.65% in particular to increase strength.    -   Mn: 0.1-0.4. Preferably the maximum Mn content is 0.35% and/or        the minimum Mn content is 0.24% or preferably 0.25%. Addition of        Mn improves in particular the bending behaviour.    -   Mg: 0.75-1, preferably, the minimum content of Mg is 0.80%        and/or the maximum Mg content is 0.90%.    -   Ti: max 0.15, preferably the minimum Ti content is 0.01% and/or        the maximum Ti content is 0.05%.    -   Cr: max 0.1 and preferably the Cr is an inevitable element or an        impurity.    -   V: max 0.1, and preferably the V is an inevitable element or an        impurity.    -   And the inevitable elements and impurities at maximum 0.05%        each, and total 0.15% maximum and the remainder is aluminium.

The casting can be made with various casting process. Continuouscasting, which is usually a horizontal casting, is possible. It is alsopreferred to use a vertical semi continuous casting, which is also knownunder the name of direct chill casting. The vertical semi continuouscasting is preferred because it more homogenous through the thickness ofthe sheet.

The ingot is homogenised, hot rolled and cold rolled into a sheet. Thesheet is solution heat treated and quenched. Preferably thehomogenization treatment of the ingot is at a temperature from 520 to560° C. during preferably from 2 to 8 hours. Preferably the hot rollingrolls the ingot to a rolled intermediate product having a thickness from3 to 10 mm. Preferably the cold rolling rolls the rolled intermediateproduct into a sheet having a thickness from 1 to 4 mm. The sheet isthen solution heat treated typically at a temperature beyond the solvustemperature of the alloy while avoiding incipient melting. Preferablythe solution heat treatment temperature is from 530° C., preferably 540°C. to 580° C. during preferably from is to 5 minutes. Quenching is thenapplied to the sheet. Water quenching is suitable with a temperatureabout 15 to 60° C., preferably 15° C. to 40° C. A pre ageing is appliedduring preferably at least 8 hours with preferably a temperature from 50to 120° C. Natural ageing is then applied. Natural ageing is defined inNF EN 12258-1 and room temperature is defined in NF EN ISO 6892-1.Preferably the duration of the natural ageing is from 72 hours to 6months.

The pre ageing step is preferably achieved by coiling of the sheet at acoiling temperature and cooling it in open air at the room temperature.

A convenient continuous annealing line device to realise the pre ageingis described by FIG. 3 . The sheet 3 is uncoiled by uncoiler 1 and goesthrough the solutionizing furnace 4 and the quenching unit 5, then thesheet 3 enters the surface treatment machine 6, which is a very usualstep for car body sheet, followed by a pre ageing oven 7 and finallycoiled on the coiler 8 in open air. At the exit of pre ageing oven 7,the sheet is therefore hot and the sheet is coiled on the coiler 2 at acoiling temperature in open air. The coiled sheet 8 is hot and is storedat ambient temperature in the plant and cools down to ambienttemperature. Pre ageing occurs during this cooling. Natural ageingstarts after the end the cooling of the coiled sheet 8, preferably thepre-ageing duration is at least 8 hours.

Preferably, the pre ageing is obtained by coiling the sheet at a coilingtemperature from 50 to 120° C., preferably from 60 to 120° C., followedby cooling the coiled sheet in open air, and its duration is 8 hours atleast.

The rolled product of the invention comprises the product obtainablewith the above method from casting to natural ageing. The temper of therolled product after natural ageing is T4.

T4 temper rolled product tensile yield strength varies less than 5 MPa,preferably 3 MPa between the tensile yield strength in the transverseand 45° directions within the same rolled product. The same sheet isdefined a rolled product made from the same ingot, same homogenization,same hot and cold rolling, same solution heat treatment, same quenching,same pre aging, same natural aging and the tensile testing samples arecut off from the rolled product as close as possible. This is a usefulproperty for part stamping.

The rolled product in T4 temper can be characterized in 6 othersspecific tempers, T8A, T8C, T8D, T6B, T6C and T8D, which estimate thematerial properties of the part.

The T8A, T8C and T8D tempers are achieved by applying on the T4 rolledproduct a 2% strain followed each by a specific heat treatment. T8Atemper uses a bake hardening heat treatment of 20 minutes at atemperature of 180° C. T8C temper uses a light and short bake hardeningheat treatment of 5 minutes at a temperature of 160° C. T8D temper usesa light and long bake hardening heat treatment of 20 minutes at atemperature of 160° C.

The T6B, T6C and T6D tempers are achieved by applying on the T4 rolledproduct a specific heat treatment. T6B temper uses a heat treatment at atemperature of 225° C. during 30 minutes. T6C temper uses a light andshort bake hardening heat treatment of 5 minutes at a temperature of160° C. T6D temper uses a light and long bake hardening heat treatmentof 20 minutes at a temperature of 160° C.

The T4 rolled product can then be formed, in particular by pressstamping, in order to obtain a shape. Optionally, the shape is aged. Theshape may be painted and bake hardened into a part at a temperature from150 to 190° C., and preferably from 170 to 190° C., during from 5 to 30minutes, preferably from 15 to 30 minutes.

An object of the invention is a part obtainable with the above methodwith the rolled product of the invention. The part can be used in a caras bodywork skin parts (or external bodywork panels) such as the frontwings, roofs, bonnet, boot or door skins, and the lining parts orbodywork structure components such as for example door, bonnet, tailgateor roof linings or reinforcements, or, preferably, spars, bulkheads,load-bearing floors, tunnels and front, middle and rear pillars, andfinally the impact absorbers or “crashboxes”.

In a first embodiment the coiling temperature is from 50° C. to 95° C.,95° C. being excluded, preferably from 60 to 95° C., 95° C. beingexcluded. The T4 temper rolled product of this first embodiment ischaracterized by a tensile yield strength lower than 165 MPa, which canbe useful for customer formability at press stamping. The T6B temperrolled product of this first embodiment, as described formally, has aminimum tensile yield strength of 345 MPa and preferably a minimumtensile yield strength of 350 MPa.

A preferred composition for the method according to the first embodimentis

-   -   Si: 0.75-1.10 and more preferably less 0.95%; Fe: max 0.4 and        more preferably between 0.15% and 0.30%;    -   Cu: 0.5-0.70 and preferably 0.5-0.65;    -   Mn: 0.1-0.4;    -   Mg: 0.75-1;    -   Ti: 0.01-0.05;    -   Cr: max 0.1;    -   V: as an impurity;    -   and the inevitable elements and impurities at maximum 0.05%        each, and total 0.15% maximum and the remainder is aluminium.

With this preferred composition and with a coiling temperature from 50°C. to 95° C., 95° C. being excluded, preferably 60 to 95° C., 95° C.being excluded, the bendability of the T4 rolled product of the firstembodiment is 0.19 maximum. This is advantageous in part forming.

A still more preferred composition of the first embodiment is

-   -   Si: 0.75-1.10 and more preferably less 0.95%;    -   Fe: max 0.4 and more preferably between 0.15% and 0.30%;    -   Cu: 0.5-0.70 and preferably 0.5-0.65;    -   Mn: 0.24-0.30 and preferably minimum 0.25%;    -   Mg: 0.75-1;    -   Ti: 0.01-0.05;    -   Cr: max 0.1;    -   V: as an impurity;    -   and the inevitable elements and impurities at maximum 0.05%        each, and total 0.15% maximum and the remainder is aluminium.

With this still more preferred composition, in conjunction with acoiling temperature from 50° C. to 70° C., preferably from 60 to 70° C.,the VDA angle of the T4 temper rolled product is greater than 125°. Thebendability of the T4 rolled product is still smaller than 0.19. Thiscan be useful in some press stamping application.

In another preferred method of the first embodiment the coilingtemperature is between 70° C. and 95° C. With this method, the T8Atemper rolled product has a minimum tensile yield strength of 275 MPa.In a more preferred method of this embodiment, the T8A temper rolledproduct has a minimum tensile yield strength of 280 MPa with a coilingtemperature between 70° C. and 95° C. and with a composition of

-   -   Si: 0.75-1.10 and more preferably less 0.90%;    -   Fe: max 0.4 and more preferably between 0.15% and 0.30%;    -   Cu: 0.65-0.8;    -   Mn: 0.1-0.4 and more preferably less than 0.24% and 0.15%        minimum;    -   Mg: 0.75-1 and more preferably less 0.95%;    -   Ti: 0.01-0.05;    -   Cr: max 0.1;    -   V: as an impurity;    -   and the inevitable elements and impurities at maximum 0.05%        each, and total 0.15% maximum and the remainder is aluminium.

In a second embodiment of the invention the coiling temperature is from95° C. to 120° C. and preferably from 95° C. to 105° C. with preferablythe composition: Si: 0.75-1.10 and more preferably less 0.90%;

-   -   Fe: max 0.4 and more preferably between 0.15% and 0.30%;    -   Cu: 0.5-0.70 and preferably 0.5-0.65;    -   Mn: 0.1-0.4 and preferably minimum 0.25% and preferably less        than 0.35%;    -   Mg: 0.75-1;    -   Ti: 0.01-0.05;    -   Cr: max 0.1;    -   V: as an impurity;    -   and the inevitable elements and impurities at maximum 0.05%        each, and total 0.15% maximum and the remainder is aluminium.

The advantage of this second embodiment is in particular the lowsensitivity of the yield strength of the part to a variation of the bakehardening treatment. The bake hardening conditions are dependent on thelocation inside the car body assembly, parts having a low sensitivity tobake hardening conditions are thus favourable because the carmanufacturer has more flexibility. This low sensitivity can be assessedby comparing properties in T6C temper to those in T6D temper and/orproperties in T8C temper to those in T8D temper which are obtained fromthe same T4 temper rolled product.

With rolled product obtained with the method of the second embodiment,the tensile yield strength of the rolled product in T8C and T8D tempersand made from the same rolled product in T4 temper, differ by less than5 MPa. The T8C and T8D rolled product samples differs only by theduration of the bake hardening, the temperature of which is 160° C.

The T6C and T6D rolled product samples differs only by the duration ofthe bake hardening the temperature of which is 160° C. With rolledproduct obtained with the method of the second embodiment, the tensileyield strength of the rolled product in T6C and T6D tempers and madefrom the same rolled product in T4 temper, differ by less than 5 MPa.

More generally, the rolled product can be heat treated with atemperature from 150 to 190° C., and preferably from 170 to 190° C.,during from 5 to 30 minutes, preferably from 15 to 30 minutes. The yieldstrength of the rolled product, heat treated at a given temperature inthe above temperature ranges, during any duration in the above durationranges, varies by less than 15 MPa, preferably 10 MPa and morepreferably 5 MPa.

More generally, the 2% strained rolled product can be heat treated witha temperature from 150 to 190° C., and preferably from 170 to 190° C.,during from 5 to 30 minutes, preferably from 15 to 30 minutes. The yieldstrength of the 2% strained rolled product, heat treated at a giventemperature in the above temperature ranges, during any duration in theabove duration ranges, varies by less than 15 MPa, preferably 10 MPa andmore preferably 5 MPa.

With the second embodiment, the T4 temper rolled product has a maximumtensile yield strength of 190 MPa. With the second embodiment, the T6Btemper rolled product has a minimum tensile yield strength of 340 MPa.With the second embodiment, the T8A temper rolled product has a minimumtensile yield strength of 280 MPa, preferably of 290 MPa.

Recyclability of any alloy is an important technical and economicalparameter. Reducing the range any element is useful in order tostrengthen recycling process as it gives predictability of the futuremelt. Reducing the maximum of the addition element is also advantageousas they can be more expensive than aluminium. Reducing Si content isadvantageous for recycling because in many alloys, this element is notonly an impurity but also detrimental to aluminium product properties.Therefore, an advantageous embodiment of the invention is to reduce theSi content to maximum of 0.95%. It is also an advantageous embodiment toreduce Fe maximum to 0.30% and/or to increase the Fe minimum to 0.15%.Another advantageous embodiment is to reduce the Cu maximum to 0.70% andpreferably to 0.65% and/or to increase the Cu minimum to 0.55%. Anotheradvantageous embodiment is to reduce the Mn maximum content to 0.35% andmore preferably to 0.30% and/or to increase its minimum content to 0.15%and more preferably to 0.25%. Another embodiment is also to reduce theTi maximum content to 0.05% and/or to increase the minimum content to0.01%. Another embodiment is to classify the V as an impurity with amaximum of 0.05%

All those combinations of alloys composition and coiling temperature ofthe invention gives many possibilities for the car manufacturer withdifferent forming properties. The car manufacturer can also optimize itsprocessing and the design of its part. The shape ageing allows a highstrength part but it requires a specific heat treatment of the shapeageing. High strength alloys are useful to lightweight part. If the partdoes not require high strength material, the car manufacturer can avoidthe shape ageing, which is advantageous to simplify the production.Hence, the invention gives flexibility to car manufacturer.

EXAMPLES

Preamble

Table 1 summarises the chemical compositions (% by weight) of the alloysused during tests. The proportion of the others inevitable elements andimpurities were lower than 0.05%, the total lower is than 0.15%, and theremainder is aluminium. Alloy G is an exemplary AA6111 alloy and alloy His an exemplary of a modified AA6056.

TABLE 1 Alloy Si Fe Cu Mn Mg Ti Cr V A 0.81 0.21 0.68 0.20 0.7 0.04<0.05 <0.05 B 0.81 0.21 0.70 0.20 0.8 0.03 <0.05 <0.05 C 0.81 0.20 0.580.20 0.7 0.03 <0.05 <0.05 D 0.80 0.20 0.58 0.20 0.9 0.04 <0.05 <0.05 E0.83 0.19 0.56 0.29 0.8 0.03 <0.05 <0.05 F 0.82 0.20 0.58 0.29 0.9 0.10<0.05 0.07 G 0.70 0.20 0.65 0.20 0.7 0.04 <0.05 <0.05 H 0.81 0.20 0.850.20 0.7 0.05 <0.05 <0.05

The rolling ingots of these various alloys were obtained by verticalsemi-continuous casting. After scalping, these various ingots underwenthomogenisation heat treatment at 540° C. during about 4 hours directlyfollowed by the hot rolling to a 5 mm intermediate rolled product. The 5mm intermediate rolled product was cold rolled to obtain sheets with athickness of 2 mm.

The rolling steps were followed by a solution heat treatment followed byquenching. The solution heat treatment was at a temperature beyond thesolvus temperature of the alloy while avoiding incipient melting. Inthis non limitating example the solutionizing temperature was 570° C.The solutionized sheet was then water quenched in a 20° C. water. Thesheet samples were coiled with 3 coiling temperatures of 100° C., 80° C.and 60° C. for a pre ageing of 8 hours followed by a natural ageing. Twonatural ageing were used: 7 days and 30 days at room temperature toobtain T4 temper rolled products.

The T4 rolled products were transformed into a T8A temper with a 2%strain and then heat treatment with a typical bake hardening heattreatment of 180° C. during 20 minutes. T8A samples were thencharacterized.

The T4 rolled product were also heat treated into a T6B temper with aheat treatment of 225° C. during 30 minutes. T6B samples were thencharacterized.

Tests Results

Tensile tests at ambient temperature were carried out in accordance withNF EN ISO 6892-1 with non-proportional test pieces, with a geometrywidely used for sheets, and corresponding to the type of test piece 2 intable B.1 of Appendix B of said standard. These test pieces inparticular have a width of 20 mm and a calibrated length of 120 mm.Tensile tests were done on rolled product in T4, T8A and T6B temper. Theresults obtained with a coiling temperature of 80° C. and 30 days ofnaturel ageing are presented in Table 2. The results obtained with acoiling temperature of 60° C. and 30 days of naturel ageing arepresented in Table 3. The results obtained with a coiling temperature of60° C., 80° C. and 100° C. and 7 days of naturel ageing are presented inTable 4.

TABLE 2 Coiling temperature 80° C. + 30 days natural ageing Measures inlong transverse direction Bending radius three-point Tensile Yieldstrength, T4 radius bending test T4 T8A T6B WRAP T4 T4 T4 Alloy MPa MPaMPa mm r/t Angle α ° Fmax N A 140 268 336 0.3 0.15 127 5303 B 152 288356 0.4 0.20 118 4911 C 138 255 339 0.2 0.10 121 4316 D 152 275 355 0.30.14 123 4972 E 149 279 353 0.3 0.15 122 4800 F 151 278 353 0.4 0.20 1154766 G 129 254 325 0.3 0.14 129 4924 H 148 270 344 0.4 0.16 115 5453

TABLE 3 Coiling temperature 60° C. + 30 days natural ageing Measures inlong transverse direction Bending radius three-point Tensile Yieldstrength, T4 radius bending test Refer- T4 T8A T6B WRAP T4 T4 T4 enceMPa MPa MPa mm r/t Angle α ° Fmax N A 140 230 334 0.3 0.15 133 5928 B149 248 352 0.4 0.20 113 5295 C 138 238 337 0.2 0.10 128 4687 D 150 245356 0.3 0.14 120 4826 E 150 241 351 0.3 0.15 135 5852 F 154 244 354 0.40.20 110 5080 G 135 221 326 0.3 0.14 133 5281 H 152 342 0.4 0.16 1165679

TABLE 4 Coiling temperature + 7 days natural ageing Alloy 60° C. 80° C.100° C. Tensile Yield Strength MPa Measures in long transversedirection, T4 temper A 142 137 B 148 149 C 133 136 D 146 149 E 154 147174 F 152 149 G 124 125 H 149 146 170

The coiling temperature is an important parameter for T4 temper tensileyield strength. At 60 and 80° C. it allows to limit the T4 tensile yieldstrength below 165 MPa which can be advantageous for car manufacturer ifit is needed to maintain stamping easiness.

Example alloys B, D, E and F, have a tensile yield strength minimum of350 MPa in T8B temper. Those example alloys have a tensile yieldstrength minimum of 275 MPa in T8A temper.

Reducing the range of Ti to maximum 0.05%, the V to an impurity of 0.05%maximum and reducing Cu to less than 0.65% is also advantageous asexemplified by alloy E and D because it reduces the bendability to 0.15,which eases the manufacturability of the component independently of thecoiling temperature.

In addition to the above reduced range of V, Ti and Cu, the optimizedrange of Mn from 0.25 to 0.35% offers with the 60° C. coilingtemperature a very advantageous 3 points bending test with a high VDAangle which is good for formability. This is exemplified by alloy E withcoiling temperature of 60° C.

Example 2

Rolled products manufactured with alloy E, with coiling temperatures 80°C. and 100° C. and after 7 days of natural ageing were used for otherstrials. Samples at both coiling temperature were split in 2 groups: inthe first group a strain of 2% was applied and the second group therewas not any strain. Then a bake hardening temperature of 160° C. wasapplied, with two different durations of 5 and 20 minutes.Those results, provided in Table 5 for a coiling temperature of 80° C.and Table 6 for a coiling temperature of 100° C., show anotheradvantageous embodiment: with a coiling temperature of 100° C., therolled product tensile yield strength is nearly independent from bakehardening duration. This is an advantageous behaviour for parts whichcan be installed in the car body assembly either at the surface or deepinside a multiple parts assembly because their yield strength remainssimilar. This offer flexibility for part design for car manufacturer.

TABLE 5 Coilling temperature 80° C. Rp0, 2 Rm Alloy temper T° C. Tps,min Strain (%) (MPa) (MPa) E T4 147 293 E T6C 160 5 0 169 310 E T8C 1605 2 215 321 E T6D 160 20 0 194 326 E T8D 160 20 2 235 333

TABLE 6 Coiling temperature 100° C. Rp0, 2 Rm Alloy temper T° C. Tps,min Strain (%) (MPa) (MPa) E T4 174 317 E T6C 160 5 0 203 336 E T8C 1605 2 249 349 E T6D 160 20 0 204 337 E T8D 160 20 2 247 346

Example 3

An ingot of the following composition was cast

An ingot with the chemical composition in table 7 (% by weight) was castusing a vertical semi continuous casting. The proportion of the othersinevitable elements and impurities were lower than 0.05%, and the totalis lower than 0.15%, the remainder is aluminium.

TABLE 7 Si Fe Cu Mn Mg Cr Ti 0.86 0.21 0.66 0.28 0.85 0.01 0.04

The rolling ingot were heated at 554° C. during 4 hours. The ingot wasdirectly hot rolled. The temperature of the ingot just before the startof hot rolling was 540° C. The thickness at the end of hot rolling was 5mm. The thickness at the end of cold rolling was 2 mm. The sheet wassplit in three in order to solutionize at three different temperatures,535° C., 544° C. and with each a different duration above 525° C.: 20s,45s and 68s. The sheets were quenched in 22° C. water. The sheets werepre aged by coiling the sheets at a temperature of 96° C. and cooling inopen air followed by a natural ageing at room temperature about 20° C.during 3 days to obtain T4 temper rolled products.

The T4 rolled products were transformed into a T8A temper with a 2%strain and then heat treatment with a typical bake hardening heattreatment of 180° C. during 20 minutes. T8A samples were thencharacterized.

The T4 rolled product were also heat treated into a T6B temper with aheat treatment of 225° C. during 30 minutes. T6B samples were thencharacterized.

Tensile tests were done in the rolling direction (L), in the transversedirection to the rolling direction (T) and direction at 45° the rollingdirection (45′).

TABLE 8 Solution HT ° C. Direction Temper YS, MPa UTS, MPa Ag % A % 535L T4 183 309 22 26 535 T T4 171 304 25 27 535 45° T4 172 302 25 27 535 LT8A 303 369 17 21 535 T T8A 293 361 17 21 535 45° T8A 297 362 17 21 535L T6B 356 380 7 11 535 T T6B 345 376 8 12 535 45° T6B 346 375 8 11 544 LT4 180 308 21 27 544 T T4 168 301 24 26 544 45° T4 170 300 25 28 544 LT8A 307 373 17 21 544 T T8A 300 366 17 21 544 45° T8A 304 367 17 21 544L T6B 356 380 8 11 544 T T6B 344 376 8 11 544 45° T6B 344 376 8 12 559 LT4 183 312 22 27 559 T T4 172 304 24 26 559 45° T4 175 304 26 28 559 LT8A 305 371 17 21 559 T T8A 298 366 17 21 559 45° T8A 302 366 17 21 559L T6B 360 382 8 11 559 T T6B 350 381 8 12 559 45° T6B 349 379 8 11

Table 8 shows the solution heat treatment is reliable to processvariation about temperature or duration to obtain the mechanicalproperties.

T4 temper tensile yield strength shows an anisotropy of less than 3 MPabetween the tensile yield strength in the T and 45° directions withinthe same rolled product as it can be seen in table 8.

Bending radius was also measured on T6B temper to check the crashbehaviour of the rolled product. Results are disclosed in table 9.

TABLE 9 Solution Bending HT ° C. Direction Temper radius r/t 535 L T6B0.889 544 L T6B 0.889 559 L T6B 1.016

1. A method for manufacturing a rolled product for automobile bodyworkor body structure, and/or a “body in white”, from an aluminium alloy,comprising: a. casting of an ingot with the following composition (% byweight): Si: 0.75-1.10; Fe: max 0.4; Cu: 0.5-0.8; Mn: 0.1-0.4; Mg:0.75-1; Ti: max 0.15; Cr: max 0.1; V: max 0.1; inevitable elements andimpurities at maximum 0.05% each, and total 0.15% maximum; remainderaluminium, b. homogenization of the ingot, c. hot rolling of the ingot,d. cold rolling into a sheet, e. solution heat treatment, quenching ofthe sheet, f. pre aging of the sheet, g. natural aging of the sheet. 2.The method according to claim 1, wherein the Cu maximum content of theingot is 0.70% and/or the Cu minimum content is 0.55%.
 3. The methodaccording to claim 1, wherein the Mn maximum content of the ingot is0.35% and/or the Mn minimum content is 0.15%, optionally 0.24% andoptionally 0.25%.
 4. The method according to any claim 1, wherein the Timaximum content of the ingot is 0.05% and/or the Ti minimum content is0.01%.
 5. The method according to claim 1, wherein V is among theinevitable elements or impurities.
 6. The method according to claim 1wherein: b. homogenization of the ingot is at a temperature from 520 to560° C. optionally during from 2 to 8 hours, and/or c. hot rolling ofthe ingot is to a thickness from 3 to 10 mm, and/or d. cold rolling intoa sheet is to a thickness from 1 to 4 mm, and/or e. solution heattreatment temperature is from 540 to 580° C. optionally from 1 s to 5minutes, and/or f. pre aging is during at least 8 hours at a temperatureoptionally from 50° C. to 120° C., optionally by coiling the sheet at acoiling temperature from 50° C. to 120° C., and/or g. natural aging isat ambient temperature, optionally from 72 hours to 6 months.
 7. Themethod according to claim 1, wherein the casting a is a vertical semicontinuous casting.
 8. The method according to claim 6 wherein the preaging is obtained by coiling the sheet at a coiling temperature from 70°C. to 95° C., 95° C. being excluded.
 9. The method according to claim 6wherein the pre aging is obtained by coiling the sheet at a coilingtemperature between 50° C. and 70° C.
 10. The method according to claim6 wherein the pre aging is obtained by coiling the sheet at a coilingtemperature above 95° C., and optionally preferably from 95° C. to 105°C.
 11. A rolled product obtainable according to the method of clam 1.12. A rolled product obtainable with the method of claim 8 wherein thetensile yield strength of the rolled product in a T4 temper is below 165MPa and wherein the tensile yield strength of the rolled product in aT6B temper is at least 345 MPa.
 13. The rolled product obtainable withthe method of claim 9 wherein the tensile yield strength of the rolledproduct in T8A temper is at least 275 MPa.
 14. The rolled productobtainable with the method of claim 10 wherein the tensile yieldstrength of the rolled product in T8C and T8D tempers and made from thesame rolled product in T4 temper, differ by less than 5 MPa, and whereinthe tensile yield strength of the rolled product in T6C and T6D temperand made from the same rolled product in T4 temper differ of less than 5MPa and/or wherein T4 temper rolled product has a maximum tensile yieldstrength of 190 MPa and/or wherein the T6B temper rolled product has aminimum tensile yield strength of 340 MPa and/or wherein T8A temperrolled product has a minimum tensile yield strength of 280 MPa,optionally of 290 MPa.
 15. The method according to claim 1 furthercomprising the following: g. Forming the rolled product, optionally bypress stamping, into a shape, h. optionally artificial aging of theshape, i. painting and “bake hardening” of the shape into a part at atemperature from 150° to 190° C. and optionally from 170° to 190° C.,during from 5 to 30 minutes, optionally from 15 to 30 minutes.
 16. Partobtainable with the method according to claim
 15. 17. A productcomprising the part according claim 16 in a car as bodywork skin parts(or external bodywork panels) optionally front wings, roofs, bonnet,boot or door skins, and/or lining parts or bodywork structure componentsoptionally door, bonnet, tailgate or roof linings or reinforcements, or,optionally, spars, bulkheads, load-bearing floors, tunnels and front,middle and rear pillars, and/or impact absorbers or “crashboxes”.